Standard WFPC2 CCD processing following
Windhorst et al. (1994b,
1994c,
1998a),
Driver et al. (1995a),
Neuschaefer & Windhorst
(1995), and
Odewahn et al. (1996)
included bias and dark-subtraction, and
flat-fielding. Photometric calibration was done using the STSDAS
On-The-Fly-Reduction (OTFR) routines as available since summer 2000.
Custom calibration, in general, does not significantly improve upon the
STScI WFPC2 pipeline, owing to the significant work that went into
building and improving that pipeline. The OTFR takes into account the
latest improvements in knowledge of the instrument every time one
retrieves data from the HST Archive.

Because the mid-UV images have extremely low sky-background levels, the
background subtraction is limited by the quality of the bias and dark
current removal. It is therefore important that the very best possible
biases and latest dark-current and hot-pixel maps are used. We paid
close attention to whether the correct dark-frames were used when
observations were taken near the monthly warm-up of WFPC2 (to
decontaminate the optics and anneal many of the new hot-pixels). The
OTFR uses the best available super-dark taken after the relevant science
images, but before the next decontamination. Hence, it is conservative
in nature - repairing more pixels than needed, but never too few. We
re-ran the OTFR on all data one month after the last images for this
project were taken (in April 2001) to incorporate the latest knowledge
on the WFPC2 data. The difference between this second run and the first
was very small, but went in the direction of removing a more appropriate
(smaller) number of pixels deemed "hot".

We co-added all images in the same filter after registration using
integer pixel shifts. Our in-house IDL routine STCombine
(Pascarelle et al. 1998;
Cohen et al. 2002)
was used to optimally remove
the signal induced by the many Cosmic-Ray (CR) hits. This routine,
optimized for the low signal domain, applies a one-sided
2-
rejection in creating the final stacked image (following
Windhorst et al. 1994a).

The Zodiacal sky-background at the North Ecliptic Pole is ~ 24.0
mag arcsec - 2 in F300W
(Windhorst et al. 1994b,
1998a)
and ~ 24.7 mag arcsec - 2 in F255W
(Cornett et al. 1994),
and is as low in the sunlit part of the
orbit as it is in the occulted part (unless the angle to the Earth's
limb becomes small). Since our WFPC2 mid-UV images are read-noise
limited, the resulting 1-orbit
1- SB-sensitivity is
25.1 ± 0.15 mag arcsec - 2 in F300W and 23.0 ± 0.15
mag arcsec - 2 in F255W on a
per pixel basis. The relation between the detected SB-level and the S/N
in a pixel is given by: S/N = 10 - 0.4
(µF300W - 25.1) and S/N = 10 - 0.4
(µF255W - 23.0) for F300W and F255W,
respectively. These mid-UV
SB-limits are consistent with the values expected from the Cycle 5-6
images in F410M and F450W
(Pascarelle et al. 1996,
Odewahn et al. 1996,
Windhorst et al. 1998a)
and the relatively red color of the zodiacal
sky-background. Taking into the account the (1 + z)4
SB-dimming and
the typical (U - I) color of galaxies at z 1-2, the
SB-sensitivity reached in the present data matches that achieved for
typical faint I
26 galaxies seen in deep HST images.

The 1-orbit 3- point
source sensitivity is 26.4 ± 0.15 mag in
F300W and ~ 24.5 mag in F255W. Hence, many of the galaxies in our
sample are resolved into their brightest star-forming regions and -
most likely - into their OB associations and young star clusters.
This is not true, however, for most of the merging/interacting galaxies
that were selected into our sample from the sample of
Hibbard et al. (2002).
Because these systems are relatively rare, they tend to be 2-3
times more distant than the bulk of our sample (see
Table 1, Col. 14),
and so are not resolved into stars.

The F300W filter has a significant red-leak, causing a fraction of an
object's flux longward of 4000Å to be detected in this mid-UV filter.
Fig. 3.10 of the WFPC2 Handbook
(Biretta et al. 2001)
shows that red-leak
portion of the QE × T curve of the F300W filter resembles the
throughput curve of the F814W filter which transmits mostly photons in
the 7000-9000Å range. Table 3.13 of the WFPC2 handbook suggests
that the red-leak is generally no more than 5% of the total F300W flux
for stellar populations dominated by stars of spectral type K3V or
earlier, although it can be as much as 10-50% of the total F300W flux
for stellar populations dominated by M0-M8V stars. Hence, even for
elliptical galaxies with K-star spectra, the red-leak is expected to be
relatively small, and for late-type galaxies dominated by young hot
stellar populations it should be almost negligible.

For realistic galaxy SEDs,
Eskridge et al. (2002a)
find that the red-leak
is typically 5-7% of the total F300W flux, and never exceeds 10% of
the F300W flux, not even in the reddest galaxy bulges. We verified this
for several red galaxies in our sample by subtracting a fraction of the
F814W images from the F300W images, after appropriately rescaling with
the relative exposure time, and making sure both images were registered
the same way. This fraction of the subtracted F814W image amounted to
7% of the total F300W flux in the brightest region of the galaxy bulge
that is presumed to be dominated by G8-K3 stars (following the red-leak
as modeled by
Eskridge et al. 2002a).

We found that for the redder stellar populations in those images no
noticeable additional structure was introduced in our F300W images at
the locations of the brightest F814W flux. To illustrate this, a very
red star is seen just above the center of both edge-on galaxies
ESO033-G22 and IC 4394 in the F814W images of Fig. 3.19 and 3.20. For
IC 4394, Fig. 4.20 shows how red this star is, where it is seen just
South of the galaxy center. These stars are saturated in the F814W
images of both galaxies, and at the corresponding locations in the
(non-red-leak corrected) F300W images of Fig. 3.19-3.20, only a very
faint red-leak flux is seen. These worst case examples show that the
apparent F300W morphology of any of our galaxies would not be
significantly affected by the red-leak in a few of the very reddest and
brightest galaxy areas in the F814W images. Such areas would have to be
significantly saturated in our F814W exposures to generate significant
red-leak in the F300W images, and none of our targeted galaxies were
saturated anywhere in the F814W images. Hence, for the current
qualitative presentation of the mid-UV images, and given that our sample
is biased toward the bluer galaxies, we have thus not corrected the
images presented in the mid-UV atlas of Section 3
for the small contamination by red-leak in the brightest and reddest
areas.

For accurate quantitative measurements of galaxy properties we will
subtract the red-leak in future papers where necessary (e.g.,
Eskridge et al. 2002a).
This will be done together with the determination of
pixel-to-pixel SEDs for each galaxy, in case there are subtle
dependencies of the red-leak on the red SED. However, to first order
there should be no such dependency, since galaxies of all spectral types
show remarkably little change in their relative SED's between
7000-9000Å.

As we expect these data to be of use beyond the scope of our immediate
science goals, we will make all images available to the community in
digital form when this paper goes to press. We do this, even though the
photometric zero-points for part of the ground-based images are not yet
established. We will update the FITS headers as new photometry becomes
available (see also
Taylor et al. 2002).
Hence, the FITS headers in the
public data base will override any values currently listed in
Table 2.
The images will be made public via ADS and also at the following public
Web-Site at ASU:

Both the HST and ground-based data will be made publicly
available at this site. The raw WFPC2 data can be obtained from the
HST Archive. Additional information regarding this survey and its
planning, the observations, and reduction procedures can be found at: